Overindulge in Greek island group (6)
I believe the answer is:
cosset
'overindulge' is the definition.
(I've seen this before)
'greek island group' is the wordplay.
'greek island' becomes 'cos' (I've seen this before).
'group' becomes 'set' (both can mean a collection).
'cos'+'set'='COSSET'
'in' is the link.
(Other definitions for cosset that I've seen before include "Pamper and spoil" , "Nanny" , "Protect overindulgently" , "Treat overindulgently" , "Fondle" .)